The increasing practical use of blue-violet semiconductor lasers has led to the practical application of the Blu-ray Disc (hereinafter, also referred to as BD) which is a high-density, large-capacity optical information recording medium (hereinafter, also referred to as an optical disc) with the same size as CDs (Compact Discs) and DVDs. A BD is an optical disc for recording or reproducing information on or from an information recording surface having an approximately 0.1 mm-thick light transmission layer, by using a blue-violet laser light source that emits laser light with a wavelength of approximately 400 nm and an objective lens with a numerical aperture (hereinafter, also referred to as an NA) of approximately 0.85.
As a previous attempt to increase capacity of optical discs such as a BD, information recording surfaces have been given multilayered structures (multilayering).
However, the multilayering of an information recording surface creates a problem in that unnecessary light (so-called stray light) reflected by an information recording surface that differs from an information recording surface that is a recording or reproducing object converges at a light receiving section of a light receiving element that detects an information signal and/or a servo signal and interferes with signal light.
In consideration thereof, in order to solve such problems, for example, Patent Literature 1 discloses appropriately arranging a luminous flux splitting pattern of a diffraction grating used for detecting an information signal and/or a servo signal and a light receiving section pattern of a light receiving element. Accordingly, since stray light reflected by a plurality of information recording surfaces does not enter each light receiving section, even with a multilayered optical disc, an information signal and/or a servo signal is not affected by interference or an offset and stable signal detection can be realized.
In addition, for example, Patent Literature 2 discloses providing a light shielding section with low transmittance in a diffraction grating (a luminous flux splitting element) used for detecting an information signal and/or a servo signal. By shielding at least a central portion of a luminous flux, since the information signal and/or the servo signal and stray light reflected by an information recording surface that differs from an information recording surface that is a recording or reproducing object do not interfere with each other, stable signal detection can be realized.
Meanwhile, reducing intervals between information tracks (track pitch) of an optical disc or, in other words, attaining a narrow track pitch is known as means for increasing capacity of an optical disc. In this case, reducing the track pitch creates a problem of increasing so-called crosstalk where a signal of an information track (adjacent track) being adjacent to an information track that is an object of reproduction leaks into a reproduced signal and the crosstalk is included in the reproduced signal as noise.
In consideration thereof, in order to solve such problems, for example, Patent Literature 3 discloses an optical disc apparatus that reduces crosstalk (hereinafter, also referred to as crosstalk cancellation) by computing output of a light receiving element that receives a luminous flux split into three in a perpendicular direction to information tracks.
FIG. 63 is a diagram showing a configuration of a light receiving section of a conventional optical disc apparatus. As shown in FIG. 63, an output signal S of a light receiving section PD that receives a luminous flux LF split into three in a perpendicular direction to an information track may be computed based on the following equation.S=k·C+R+L, 
where C denotes a signal from a central portion region, R denotes a signal from a right portion region, L denotes a signal from a left portion region, and k denotes a constant. Accordingly, crosstalk is reduced.
A further increase in the capacity (density) of optical discs such as a BD requires both multilayering and attaining a narrow track pitch.
However, none of the optical disc apparatuses disclosed in Patent Literature 1, 2, and 3 assumes a high-density optical disc having been multilayered and having attained a narrow track pitch. In addition, all of the optical disc apparatuses disclosed in Patent Literature 1, 2, and 3 completely fail to consider the effect of stray light reflected by an information recording surface that differs from an information recording surface that is a recording or reproducing object on the performance of crosstalk cancellation.
In other words, there is a problem in that interference between stray light caused as a result of multilayering and signal light prevents crosstalk cancellation from being achieved in a stable manner.
Patent Literature 1: Japanese Unexamined Patent Publication No. 2008-135151
Patent Literature 2: Japanese Unexamined Patent Publication No. 2008-198336
Patent Literature 3: Japanese Unexamined Patent Publication No. H5-242512